The present invention relates to a planar light source unit for emitting a linear light beam, which unit is used in a display having a backlight device, front light device, or a light guide.
In recent years, the liquid crystal display device having the backlight device is used in the notebook personal computer, desktop personal computer, and others.
FIG. 2 is a sectional side view showing a conventional planar light source unit. In the drawing, the planar light source unit comprises a transparent light leading plate 1 made of a material capable of effectively transmitting light, such as acrylic resin, and formed into a rectangular parallelepiped, and an LED package 3 having a LED 2 which is suitable for rendering the light source compact. The light leading member 1 has a light discharge surface 1a. On a lower plane 1b, a plurality of small prisms 4 are formed, so that light is reflected from the prisms 4 to the light discharge surface 1a. The prisms 4 are formed when molding the light leading plate 1.
On the underside of the plate 1, a reflection plate (not shown) made of a white sheet is attached so as to reflect the light passing through the light leading plate 1.
The light from the LED 2 enters the light leading plate 1. The light is reflected at the upper light discharge surface 1a and at the lower plane 1b where there are no prisms, as the light is transmitted further into the light leading plate 1 without loss. The light is further reflected upward at the grooves 4 and discharged from the light discharge surface 1a. The light passing through the lower plane 1b is reflected by the reflection plate so as to be discharged through the light discharge surface 1a. Thus a liquid crystal display panel (not shown) provided above light leading plate 1 is irradiated from the back of the panel.
In FIG. 2, the solid lines show the optical paths of light beams entering the light leading plate 1, reflected at the prisms 4, and discharged out of the plate 1. The dotted lines show a range in which the light beams cannot be utilized.
The conventional light leading plate 1 has the following problem. FIG. 3 is a graph showing the directivity of an LED. The graph indicates that the luminance is the highest in the area opposite the light source and decreases at both sides thereof, thereby forming an oval curvature. Namely, although the light beams in the range between the dotted lines in FIG. 2 have the largest luminance, they cannot be used. Hence the exiting luminance of the light leading plate cannot be increased, thereby increasing the power consumption of the LED.
An object of the present invention is to provide a planar light source unit having a light leading plate where the light beams from a light source can be effectively transmitted through a light leading plate, and which can uniformly irradiate a light discharge surface of the light leading plate, thereby increasing the luminance of the light beams and decreasing the power consumption.
According to the present invention there is provided a planar light source unit comprising a transparent light leading plate, the light leading plate having a light discharge surface at an upper surface, and a lower plane, an incidence surface formed at a side of the light leading plate, a light source provided opposite to the incidence surface, a first prism provided in the light leading plate at a position opposite to the light source.
The planar light source unit further comprising a plurality of second prisms provided on the lower plane of the light leading plate.
The first prism has an isosceles triangular sectional shape, and an apex of the isosceles triangle is directed to the light source.
One of two equal sides of the isosceles triangle is faced to the light discharge surface and the other is faced to the lower plane.